1. Field of the Invention
The present invention relates to a load driving apparatus which is provided with a high voltage power source and which reduces voltage from the high voltage power source to supply the reduced, voltage to a load when driving the load.
2. Description of the Related Art
In recent years, a system, which uses a high voltage power source of 42V connected to a motor/generator advantageous in power consumption, has been developed to drive various loads mounted on an automobile.
However, there are loads such as lamps or CPUs mounted on the automobile to which a high voltage can not be applied. For such a load, a DC/DC converter is used to convert a high voltage to a low voltage of 14V which has been conventionally regulated.
However, the DC/DC converter is an apparatus whose price becomes more expensive according to increase in power to be employed. On the other hand, a load driving apparatus disclosed in Japanese Patent Application Laid-Open (JP-A) 05-168164 is constituted by a lamp load 102, a switching element 103, a variable duty PWM (Pulse Width Modulation) control circuit 104, a voltage fluctuation detecting circuit 105 and a switch 106 to a high voltage power source 101 in series, as shown in FIG. 1.
In the load driving apparatus, the switch 106 is turned in ON state, the switching element 103 is driven according to a PWM control performed in the variable duty PWM control circuit 104 for driving the lamp load 102 by means of the high voltage power source 101. Here, a PWM control means controlling a duty ratio which is a ratio of a time period where power is supplied to the lamp load 102 and a time period where power is not supplied to the lamp load 102, referring to fluctuation of a voltage supplied to the lamp load 102, and detected in the voltage fluctuation detecting circuit 105.
Therefore, the high voltage is converted the low voltage by means of the PWM control. As a result, the lamp load 102 which is a low voltage load driven by a low voltage can be driven by a high voltage, performed the PWM control, from the high voltage power source 101.
Incidentally, since all the loads mounted on an automobile can not be driven by means of the PWM control, the above-described lamp 102 and the like are driven by means of the PWM control and the other loads are driven by means of a DC/DC converter which lowers a high voltage from the high voltage power source.
Thus, since a power supplied to the lamp load 102 can be handled in the load driving apparatus, a power handled in the DC/DC converter can be reduced correspondingly, so that an inexpensive DC/DC converter can be employed.
However, in the conventional load driving apparatus, for converting the high voltage (42V) to the low voltage (14V) by means of the PWM control, when the power supplying time of the duty ratio is set to xe2x80x9c1xe2x80x9d, the time where power is not supplied must be set to xe2x80x9c9xe2x80x9d or so. In this case, the value of current occurring at a time of start of power supplying reaches about three times that occurring when the low voltage is used.
Particularly, when a rush current generates at a time of start of driving a load, the value of current flowing in the load reaches several hundreds [A]. Therefore, in the conventional art, when a load is driven while PWM control is being performed, it is necessary to use a switching element (for example, an FET (Field Effect Transistor)) which can withstand a rush current or increase the value of current supplied at a time of start of driving the load gradually.
Also, it is necessary to use an expensive switching element with a large size which can withstand a rush current, as compared with a switching element corresponding to a steady current.
In an approach where the value of current supplied at a time of start of driving a load is increased gradually, there is a problem that a circuit is complicated, heat generation of an FET in a transient state occurs and start of driving a load is delayed. Specifically, in case that a lamp is driven as a load, there occurs a problem that it takes much time to turn the lamp ON.
The present invention has been proposed in view of the above-described circumstances, and it is an object of the present invention to provide a load driving apparatus which reduces a rush current occurring at a time of start of driving a load.
It is another object of the present invention to provide a load driving apparatus which realizes to reduce costs of apparatus and simplify a circuit.
In order to achieve the above object, there is provided a load driving apparatus comprising: a first switching element which is connected to a first power source supplying a first voltage and connected to a load; a second switching element which is connected to a second power source supplying a second voltage lower than the first voltage and connected to the load; current detecting means which detects a value of current flowing in the load; and control means which is connected the first switching element, the second switching element, and the current detecting means to control a voltage to be supplied to the load, wherein the control means turns the second switching element into ON state at first, continues to supply the second voltage from the second power source to the load while the value of current detected by the current detecting means is more than a predetermined value, and turns the second switching element in OFF state and drives the first switching element to supply the first voltage from the first power source to the load once the value of current detected by the current detecting means becomes below the predetermined value.
According to the present invention, the second switching element is turned in ON at first and the second voltage lower than the first voltage is being supplied to the load while the value of current detected by the current detecting means is more than the predetermined value, and the second switching element is turned in OFF and the first switching element is driven to supply the first voltage from the first power source to the load once the value of current becomes below the predetermined value. Therefore, the second voltage lower than the first voltage is supplied to the load at a time of start of driving the load. As a result a rush current generated at a time of start of driving the load is reduced.
In a preferred embodiment of the present invention, direct current voltage converting means which is connected to the first power source and which lowers the first voltage is further provided.
According to the embodiment, the direct current voltage converting means lowers the first voltage. Therefore, overcurrent does not flow in the load and a complex control for suppressing overcurrent or switching element having a large allowable current value is not required. As a result, it is realized to reduce costs of apparatus and simplify a circuit.
In a preferred embodiment of the present invention, the first voltage from the first power source is converted to a pulse voltage by means of the direct current voltage converting means having PWM (Pulse Width Modulation) control and is supplied to the load.
According to the embodiment, since the first voltage is converted to the pulse voltage by the PWM control, a reduced first voltage can be supplied to the load. Therefore, it is realized to reduce costs of apparatus and simplify a circuit.
In a preferred embodiment of the present invention, the current detecting means detects a value of current flowing in a resistor provided between the second switching element and the load.
According to the embodiment, the value of current flowing between the second switching element and the load can be obtained from a voltage drop occurring at the resistor.
In a preferred embodiment of the present invention, the load is a lamp.
According to the embodiment, even when the load is the lamp, a rush current generated at a time of start of driving the lamp can be reduced and it is realized to reduce costs of apparatus and simplify a circuit.
In a preferred embodiment of the present invention, the predetermined value is set to a value indicating that the load has become a steady state.
According to the embodiment, as the predetermined value is set to the value indicating that the load has become a steady state, the second voltage, lower than the first voltage, is supplied to the load before the load becomes a steady state, and the first voltage is supplied to the load once the load becomes a steady state. Therefore, a rush current generated at a time of start of driving the lamp can be reduced and it is realized to reduce costs of apparatus and simplify a circuit.